Amplifier for producing a current dependent on the output voltage

ABSTRACT

A high precision power supply generates voltage with an accuracy of at least thirty parts per million and is comprised of a digital section interconnected with an analog section by pulse transformers. Numbered keyboard switches in the digital section select a desired output voltage which is displayed numerically by electronic digital display tubes. A shift register in circuit with number storage means enables any one of the displayed numbers to be changed without changing any other displayed number. The analog section includes a stable reference voltage generator, electronic switch, filter and a high gain amplifier which produces an output voltage corresponding to the displayed numbers. The electronic switch passes proportional parts of the reference voltage to the amplifier via the filter under control of pulses received from the digital section. The amplifier is capable of producing current in one of the two ranges depending upon the output voltage.

United States Patent [191 Smyth 1 3,736,519 [451 May 29,1973

[54] AMPLIFIER FOR PRODUCING A CURRENT DEPENDENT ON THE OUTPUT VOLTAGE[75] Inventor: Robert C. Smyth, Huntington I Beach, N.Y.

[73] Assignee: Digital Apparatus Corporation,

Deer Park, NY.

[22] Filed: Jan. 14, 1972 211 App]. No.: 217,986

Related Application Data [63] Continuation of Ser. No. 23,30l, March 27,1970,

abandoned.

[52] U.S. Cl. 1330/18, 330/22, 330/29 [51] lntyCl. ..H03t 3/42, l-l03g3/30 [58] Field of Search ..330/18, 22, 40, 70

[56] References Cited I UNITED STATES PATENTS 3,484,709 l2/l969 Kabriek..330/l8 X Primary Examiner-Roy Lake Assistant Examiner-James B. MullinsAttorney-Edward l-l. Loveman [57] ABSTRACT A high precision power supplygenerates voltage with an accuracy of at least thirty parts per millionand is comprised of a digital section interconnected with an analogsection by pulse transformers. Numbered keyboard switches in the digitalsection select a desired output voltage which is displayed numericallyby electronic digital display tubes. A shift register in circuit withnumber storage means enables any one of the displayed numbers to bechanged without changing any other displayed number. The analog sectionincludes a stable reference voltage generator, electronic switch, filterand a high gain amplifier which produces an output voltagecorrespondingto the displayed numbers. The electronic switch passesproportional parts of the reference voltage to the amplifier via thefilter undercontrol of pulses received from the digital section. Theamplifier is capable of producing current in one of the two rangesdepending upon the output voltage.

5 Claims, 6 Drawing Figures INPUT FROM FILTER Patented May 29; 1973 4Shoots-Shoot 4 smww m at m @I F31:

. 9w 0mm m5 3 2 mm mm, 0% 2 mu i Q INVENTOR ROBERT c. SMYTH ATTORNEY \T2, 9 Q X mm T E M M W AMPLIFIER FOR PRODUCING A CURRENT DEPENDENT ON THEOUTPUT VOLTAGE This application is a continuation of application Ser.No. 23,301, filed Mar. 27, 1970 and now abandoned.

This invention relates to a power supply apparatus capable of producinga wide range of DC voltages with extreme accuracy, and more particularlyrelates to a power supply apparatus which uses pulse width modulation ina voltage regulator circuit.

While precision power supplies have been known heretofore they havesuffered from a variety of difficulties and disadvantages. One objectionhas been the very high cost and complexity of apparatus capable ofgenerating precision voltages. Another difficulty has been the inherentlack of stability of the power supply and the tendency to drift andchange output over a period of time. The present invention is directedat overcoming the above and other difficulties and disadvantages ofprior power supplies and at the same time provide a more reliable,simpler, less expensive, more easily operable precision apparatus. Theapparatus of the invention consumes less powerthan prior precision powersupplies'and is smaller and lighter in weight.

In accordance with the invention, the apparatus uses pulse widthmodulation in a voltage regulator circuit. That is, a regulated directcurrent voltage is obtained by' feeding an output responsive, variablewidth pulse train of a given polarity to a filter circuit which therebyprovides a direct current output proportional to the average amplitudeof the pulse width modulated pulse train. By varying the width of thepulses, the average value of the direct current output voltage can becontrolled. This method of voltage regulation is relatively efficientsincethere is no appreciable power dissipated in resistors or otherdissipation elements as in the case of other methods of voltageregulation heretofore practiced.

Accordingly the apparatus of the instant invention is capable ofgenerating voltages in the range of 1,000 volts with such precision thatthe apparatus may be used as a secondary standard or calibrator forcalibrating other electronic equipment. The apparatus is operated bypush-buttons and provides a direct digital display of voltage beinggenerated. Output voltages may be displayed with six significant digits.The apparatus has two basic sections, a digital section and an analogsection. The digital section includes manually operable push-buttonvswitches which actuate a shift register and digital 'store via adecoder. Electronic digital display tubes are connected in circuit withthe store to indicate the numerical value of the voltage generated inthe ap-. paratus. In a preferred embodiment of the invention, thisvoltage is generated selectively in one of four ranges in increments ofone part per million in each range. The digital section further includescircuits which drive electronic switches in the analog section. A singlehigh precision thermally stabilized voltage generator in the analogsection is connected to the disadvantage of employing multiple resistorsin each range aside from the high cost is the inherent lack of stabilityand reliability they cause. For example, when as many as twenty-fourresistors have to be switched, the switch contacts become noisy andresistive and this changes the output, so that the high precisioncharacter of the device is soon lost. The present invention avoids theseundesirable conditions by avoiding switched resistors in each rangewithin each of the four ranges. In the present invention switching isaccomplished electronically by varying the gain of the amplifier so thatthe apparatus produces designated stable, precision voltages withnegligible electrical noise. The apparatus produces the designatedvoltages with positive or negative polarity and with differentselectable output currents.

It is therefore a principal object of the invention to provide a highprecision power supply having digital and analog sections, the digitalsection being operated by numerical pushbutton switches and providing adigital display of voltage generated; the analog section employingelectronic switching in conjunction with a stable internally generatedreference voltage to drive a high gain, multiple range, feedbackamplifier.

A further object is to provide a high precision power supply employing athermally stabilized internal reference voltage generator to driveelectronic switching circuits employing field effect transistors.

Still another object is to provide a high precision power supply with ahigh gain, multiple range output amplifier having only two fixedresistors determining the output of each range.

These and other objects and many of the attendant advantages of thisinvention will be readily'appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram of high precision power supply apparatusembodying a preferred form of the invention, showing digital and analogsections of the apparatus;

FIG. 2is a graphic diagram of pulses generated in the apparatus, and isused in explaining the invention,

FIG. 3 is a diagram of a shift register employed in the digital sectionof the apparatus;

FIG. 4 is a diagram of'the analog section of the apparatus;

FIG. 5 is a diagram of the amplifier employed in the apparatus; and

FIG. 6 is a front view of a control panel forming part of the apparatus.

Referring now to the drawings, wherein like reference numerals designatelike parts throughout the figures thereof, there is shown in FIG. 6 acontrol panel 10 of the apparatus which has a digital display window 12in which appear six numerals 14 on the faces of six electronic displaytubes 16. The numerals may range from "0 to 9 on each tube and thenumber displayed indicates the voltage being generated by the apparatusat a pair of output jack terminals 18. Polarity pushbutton selectorswitches 20 and 21 determine the polarity of the D.C. voltage producedat the output terminals l8 and this polarity is indicated by a plus orminus symbol 23 on a display tube 24. Operation of a selector switch 27results in the production of0 output voltage, i.e., a short circuit isproduced across the output terminals I8 without changing the numericaldisplay 14. Appropriate plugs can be inserted into the jack terminals 18for applying the generated precise output voltage to an external circuitfor any desired purpose. A keyboard 25 on the panel 10 is provided with10 pushbutton switches 26 designated respectively to 9. When thesepushbuttons are pushed in sequence to select a desired output voltage,the voltage will be numerically displayed at window 12. In the presentembodiment, six numerals 14 with a decimal point 15 will appear.However, provision can be made for more or less numerals according torequirements by employing the principles of the invention describedbelow. Four pushbutton switches 30 operate as range selectors whereinthe operation of any one switch will set the apparatus to producevoltages in one of four designated ranges 0-1, 0-10, 0 -100 or O-l,000volts. A CLEAR switch 32 will cause clearance of all numbers appearingin the window 12. Six switches 34 marked 10" to 10 can be selectivelyoperated for enabling the change of any one of the six numbers 14 andthe corresponding output voltage. After any one of the switches 34 isoperated, any one of the switches 26 may be operated to obtain displayof a corresponding digit on the selected tube 16. NORMAL switch 36resets the switches 34 to open condition. A power cable 38 may beconnected to any AC convenience outlet for applying power to theapparatus.

Referring now to FIG. 1, the electronic system is comprised of twosections, a digital section 100 and an analog section 200. The digitalsection is the control section and is programmed by switches on thepanel as hereinbefore described. The analog section 200 obtains signalsfrom the digital section which actuate an electronic switch 204 togenerate precision voltages. The analog section receives spike pulses Pand P" from the digital section 100 via two small ferrite pulsetransformers 190, 192. This is the only direct electrical connectionbetween the two sections. There is no other and for this reason eachsection has its own power supply 180 and 211. This is an importantfeature of the present invention and insures stability and independenceof operation in the two sections.

In digital section 100, the 10 decimal switches 26 designatedrespectively 0 to 9,, as shown in FIG. 6, are selectively operated. Adecoder 102 is connected to the switches via lines 103. From the decoder102 lines 104 convey pulses in a conventional binary l, 2, 4, 8 code inparallel to each one of a plurality of digital stores 106. Six storesare shown in FIG. 1, but more or less stores may be provided accordingto requirements. Numbers placed in the stores 106 remain there untilthey are cleared by operation of the CLEAR switch 32 connected incircuit with shift register 110. Electronic numerical display tubes 16generally known as nixie tubes are connected to the six storesrespectively.

The shift register 110 is indicated diagrammatically in FIG. 3. Thisregister has six stages designated 81-86 and each is a conventional J-Kflip-flop of the type wherein the output at-terminals Q Q assume logicalstates that were present at inputs J and K at the time the flip-flop isshifted. The stages are successively shifted by pulses generated by eachof the first six actuations of a pushbutton switchv 26 and furtheractuations thereof will not effect changes in the state of the stages.Operation of the CLEAR switch 32 connects a ground potential (via lead152) to the 0' output of flip-flop S1 and to the 0 output of flip-flopS2 through S6 (via lead 153) such that the first stage S1 is at alogical l and all other stages are at logical 0. At the same time, tubes16 all display 0. Thereafter, actuation of any selected switch 26 causesa shift pulse and a code number pulse to be generated by decoder 102.The shift pulse is applied via line 150 to the N inputs of each of thestages, and the coded number pulse is passed to all of the stores 106.The first shift pulse shifts stage S1 to a logical 0 and causes atransfer pulse to pass via the first one of lines 114 to the first store106 which is thereby locked and prevented from changing the value of thefirst coded number pulse trapped therein by subsequent coded numberpulses passed by the decoder 102. At the same time, stage S2 of theshift register shifts to a logical l Thereafter, as any selectedpushbutton decimal switch 26 is actuated, the next succeeding stage ofthe shift register 110 shifts to a logical 1" while the preceding stageshifts to logical 0. The entire numbers stored in stores 106 anddisplayed by tubes 16 remain until the CLEAR switch 32 is againoperated.

Although the hereinabove described shift register structure andoperation is fairly conventional, the register is provided with novelmeans whereby anyone of the digits of the selected number may beselectively changed. Thus, at times it may be desirable to change onlyone of the digits of a selected and displayed num-- her for example921.736 to 921.836. In prior devices, it is necessary to recycle theentire register by actuating the CLEAR switch and entering the newnumber as previously described. In this novel apparatus any digit may bechanged to another digit by merely depressing the pushbutton '34 whichrelates in the digit to be changed andv then depressing a selectedpushbutton switch 26. For example, if it is desired to change the fourthdigit (7) of the number 921.736 illustrated in FIG. 6, then thepushbutton 340 is depressed to set stage S4 to a logical l and open thefourth store. That is, a ground potential, applied to the reset Rterminal by the actuation of pushbutton 34c sets stage S4 to a logical land thereafter actuation of the numeral 8 of pushbutton switches 26 willgenerate from the decoder 102 a coded number pulse which is applied tothe open store 106s as hereinbefore described. The shift pulse which isalso generated by decoder 102 will not shift the stage S4 flip-flopinasmuch as the ground potential applied by the actuation of pushbutton340 to the reset R terminal has not been removed. If another pushbutton34 is now depressed e.g., 34b, the S4 stage will revert to a logical 0state since the ground potential is now on the set S terminal and atransfer pulse on line 114 will lock the coded number pulse in store1060 and, of course, the S5 stage will revert to a logical l state. Ifit is desired to change all the digits of a number, then the normalswitch 36 must be depressed to remove ground potential from switch 34and then the CLEAR switch 32 must be actuated to set the stage S1 tological 1" and the remaining stages S2 through S6 to logical 0'? ashereinbefore described.

Referring again to FIG. 1, it will now be understood that six codeddigits can be stored in succession in the six stores 106 and that anyoneof the digits in the six stores can be changed without changing theother digits in the stores. Connected to the respective stores is acomparator 108 having six comparison stages C1-C6. This circuit is adynamic device and compares fixed numbers stored in the stores 106 withnumbers produced continuously by a three-decade counter 110 pulsed by aclock pulse generator 112. The counter continuously produces 999 pulsesin succession and then returns to zero. These pulses are applied vialine 111 to all comparison stages and at instants of coincidence,coincidence pulses are passed to AND gates 117 from comparitors C1-C3and to AND gate 118 from comparitors C4C6. The outputs from AND gates116 and 118 actuate flip-flops 120, 122 respectively which pass spikepulses to transformers 190 and 192 respectively. Spikes pulses P and P"in turn' are applied via flip-flops 191, 193 to electronic switchcircuit 204 in analog section 200.

In analog section 200 as shown in FIG. 1, there is a reference voltagegenerator 202 which generates a single fixed D.C. reference voltage.This voltage is for practical purposes set at 8.4 volts. The referencevoltage is applied between points B and C-of an electronic switchingcircuit 204 which, although comprised of two switches 205 and 207, issymbolically shown in FIG. 1 as comprising a single pole switch 206movable between reference voltage point B and ground point C. Theswitches 205 and 207 are, as shown in FIG. 4, comprised of field effecttransistors VTS, VT6, VT7 and VT8, the operation of which will be moreclearly described below. The electronic switching is actuated by spikepulses P and P obtained from the digital section' low pass filter 208 toa high gain multirange amplifier 210. Power supply 211 energizes thevarious units and is independent of power supply 180 in the digitalsection 100.

The wave form'W of voltage applied to the filter is shown in FIG. 2. Thereference voltage is applied to the filter for time duration t whichoccurs during a portion of time period t,,. When point A is grounded topoint age is applied to the filter. The mean voltage B can be expressedas B V t,/t,,; since V is the constant reference voltage (8.4 volts),and since t, which represents the switching period can be maintainedconstant, then the mean voltage applied to the amplifier will varylinearly with time Thus, by varying time i.e., the time duration betweenapplications of pulses P and P", it is possible to vary the mean voltageoutput B of the electronic switch between V and 0. The filter 208smooths out ripples in the applied voltage so that pure D. C. is appliedto amplifier 210. The amplifier in turn amplifies the input voltage inany one of four ranges 0-1, 0-10, 0-100 and 0-1,000 volts depending onthe setting of switches 30 at panel 10. The voltage range is determinedby the feedback through one of resistors 212, depending on which one isselected for the desired amplification range. The voltage output for anyone range is therefore determined only by one of resistors 212 andresistor 214 connected between ground and the amplifier input. Thevoltage output of the amplifier appears at terminals 18 located at panel10. It should be understood that the voltage output obtained atterminals 18 is varied by changing the voltage input to the amplifier.This is accomplished by the electronic switch 204. This contrasts withconventional power supplies known in the prior art where voltage outputis varied C, then the voltage applied to the filter 208 has zeroamplitude. During time periods t,, the reference volt-' by changing thevalues of resistance at the output in each voltage range. In the presentapparatus, by contrast, the resistance remains constant for each voltagerange, and only the input voltage to the amplifier is electronicallyvaried by electronic switching. Thus, the instability and inaccuracycaused by rotary switching employed in conventional variable powersupplies is completely avoided. Amplifier 210 produces voltage of onepolarity but by inverting the output voltage the polarity with respectto ground can be reversed. This is a valuable convenience feature foruse in calibrating other instruments.

FIG. 4 shows schematically further details of the analog section 200. Inthis section is the reference voltage generator 202. A zener diode CR2which generates the reference voltage is maintained in an oven 250 at aconstant temperature of about C. The even is heated by a solid stateheater 252 energized by power supply 211. This power supply alsosupplies a DC voltage which is applied to a Zener diode CR1. TransistorsVTl, VT2, VT3 and VT4 are connected in a loop or bridge circuit withzener diode CR1 and feed a constant current to the zener diode CR2. Thisconstant current is required in order that zener diode CR2 can producethe fixed reference voltage indicated as 8.4 volts D.C., which isapplied to the electronic switch 204. Transistor VT3 which is an emitterfollower may tend to vary in internal resistance due to aging, heatingor other causes and this would tend to vary the current fed to zenerdiode CR2. To compensate for this, transistor VT4 is provided, which isidentical in characteristics to transistor VT3. Identical resistors R1and R2 carry the same current and are connected in circuit withtransistors VT3, VT4 respectively. The bases of the two transistors areconnected together. Any drift which occurs in one transistor also occursto substantially the same extent in the other transistor. As a result,the effect of drift is cancelled and the current fed to zener diode CR2remains constant. Similar drift compensation or cancellation isaccomplished between identical transistors VT! and VT2 connected base tobase with substantially no base current and constant collectoremittercurrent.

As hereinabove mentioned, the electronic switch 204 is comprised ofswitches 205 and 207 which are identical and are controlled respectivelyby the flip-flops 191 and 193. The signals from the flip-flop 193represent the first three digits and the signals from the flip-flop 191represent the last three digits of the selected number. Thus, outputsfrom the switches 205 and 207 (FIG. 4) are combined in a resistivenetwork wherein the resistor 240 has a value 1000 times greater than theresistor 242. Since both switches 205 and 207 are identical, only theoperation of switch 205 will be hereinbelow described in detail.

The constant reference voltage is applied between a drain 259 of a fieldeffect transistor VT 5 and a ground line 264. Field effect transistorVT6 has one electrode 260 connected to an electrode 257 of transistorVTS and the other electrode 262 connected to the ground line 264.Transistors VT5 and VT6 have only two operating states. When thereference voltage is to be applied to the filter 208 there is aninfinite resistance between the electrodes 260 and 262 and a low finiteresistance between the electrodes 259 and 257. When the electronicswitch 205 is to be grounded, there is a low finite resistance betweenelectrodes 260 and 2620f transistor VT6 and an infinite resistancebetween the electrodes 259 and 257 of VTS. Thus, when the VT5 transistoris conducting, the VT6 transistor is off and a proportional part of 8.4volts, the reference voltage, is applied to the filter 208, and when VTSis off and VT6 is conducting, zero voltage is applied to the low passfilter 208. The switch 207 operates in a very similar fashion.

The low pass filter 208 which is used to produce a pure D.C. signal hasa plurality of resistors 266 connected in series with input 268 ofamplifier 210, and a plurality of capacitors 270 connected in parallelbetween the resistors and ground line 264. The ground line is connectedto the other input 272 of the amplifier 210. Output terminals 18 of theapparatus are con nected to the output of the amplifier.

FIG. 5 shows the basic circuit of amplifier 210 which is controlled bythe input from filter 208 through a resistor R25. The amplifier 210 isrequired to produce output variable between and 1,000 volts and also amilliampere current above 150 volts and a 50 milliampere current below150 volts. Rather than use an amplifier comprised of single load ordrive transistor to produce the voltage and a single output transistoras emitter follower or impedance matching, there is shown in thepreferred embodiment of FIG. 5 two groups of series connectedtransistors Q8 through Q12 and Q13 through Q17, each transistor havingan equal voltage rating less than the maximum 1,000 voltage output ofthe amplifier; for example, each transistor is rated at 300 volts. Thistype of transistor circuit is preferable inasmuch as the total cost ofthe ten transistors rated at 300 volts each is substantially less thaneven a single 1,000 volt transistor. The five drive transistors Q8through Q12 are connected in series with each other, thereby sharing theload equally from 0 to 1,000 volts. The voltage applied at P1 variesbetween 0 and 1,000 volts. Five resistors R15 through R19 are connectedto the respective bases of transistors Q13 through Q17. The total outputvoltage of the amplifier 210 appears across the chain of outputtransistors Q13 through Q17 which share the voltage equally and ineffect constitute a first impedance follower. The current output fromthe amplifier of 5 milliamperes is determined by a zener diode CR12 andan output resistor R20. A second emitter-follower is comprised of atransistor Q18 which is controlled by the output voltage taken fromresistor 20. A diode CR19 is connected to a power supply 211 whichprovides 150 volts to the collector of transistor 018. Current outputfrom transistor Q18 of 50 milliamperes is limited by resistors R21 andzener diode CR13 in the same fashion as the current output fromtransistor Q17. In operation, when the voltage applied to transistor Q18is less than 150 volts, the transistor Q18 will pass 50 milliamperes ofcurrent. However, when the voltage applied from the resistor R19 isabove 150 volts, the base collector of transistor Q18 stops conductingand the diode CR19 becomes reverse biased so it does not conduct.Transistor Q18 then merely becomes a base emitter diode in effect andthe output of the amplifier will be reduced to the 5 milliamperes ofcurrent as determined by resistor R20 and the diode CR12. This then is asimple and novel amplifier which will produce two output current rangesdependent only on the output voltage.

It should be understood that the foregoing disclosure relates to only apreferred embodiment of the invention that that it is intended to coverall changes and modifications of the examples of the invention hereinchosen for the purpose of the disclosure, which do not constitutedepartures from the spirit and scope of the invention.

The invention claimed is:

1. An amplifier for producing a plurality of voltages at two differentcurrents comprising:

a signal source,

a first source of DC voltage,

a multiplicity of series connected drive transistors, each of which hasa base, said base from the first one of said transistors being connectedto said signal source and each of said bases from the other transistorsbeing coupled in parallel to said first source of DC voltage, saidtransistors producing said voltages at a terminal, first and secondimpedence means connected to said terminal, and second source of DCvoltage, said second source having a voltage magnitude substantiallydifferent than the magnitude of voltage from said first source, saidsecond source of voltage connected to said second impedence means,whereby said voltages will be at one of said currents when said voltagesare the same or below said voltage from said second DC voltage source,and said voltages will be at the other of said currents when saidvoltages exceed said voltage from said second DC voltage source.

2. Apparatus as defined in claim 1 wherein said first and said secondimpedence means includes respectively a first and a second means forlimiting the current and wherein said first current limiting meanslimits the current substantially different from the current limited bysaid second means for limiting current.

3. Apparatus as defined in claim 1 wherein said first impedence meansincludes a multiplicityof serially connected transistors and said secondimpedence means includes a" single transistor.

sistors has a voltage rating which is equal.

k l I

1. An amplifier for producing a plurality of voltages at two differentcurrents comprising: a signal source, a first source of DC voltage, amultiplicity of series connected drive transistors, each of which has abase, said base from the first one of said transistors being connectedto said signal source and each of said bases from the other transistorsbeing coupled in parallel to said first source of DC voltage, saidtransistors producing said voltages at a terminal, a first and secondimpedence means connected to said terminal, and a second source of DCvoltage, said second source having a voltage magnitude substantiallydifferent than the magnitude of voltage from said first source, saidsecond source of voltage connected to said second impedence means,whereby said voltages will be at one of said currents when said voltagesare the same or below said voltage from said second DC voltage source,and said voltages will be at the other of said currents when saidvoltages exceed said voltage from said second DC voltage source. 2.Apparatus as defined in claim 1 wherein said first and said secondimpedence means includes respectively a first and a second means forlimiting the current and wherein said first current limiting meanslimits the current substantially different from the current limited bysaid second means for limiting current.
 3. Apparatus as defined in claim1 wherein said first impedence means includes a multiplicity of seriallyconnected transistors and said second impedence means includes a singletransistor.
 4. Apparatus as defined in claim 2 wherein said first andsecond current limiting means each comprises a zener diode connectedbetween the base of a transistor and the output and a resistor coupledbetween the emitter and the output.
 5. Apparatus as defined in claim 3wherein each of said drive transistors and each of said impedencetransistors has a voltage rating which is equal.